1. Field of the Invention
This invention relates to a method and apparatus for treating an ultrahigh molecular weight, high strength polyolefin material, such as fiber, film or tape, to improve its adhesive bond to resin, particularly polymeric matrix resins such as epoxy, vinyl ester, polyester, polyurethane, polyolefin or thermoplastic rubber, which it can reinforce, preferably for composite end use. More particularly, the present invention is a method and apparatus for the continuous photografting of monomers onto polyolefin material to improve its interfacial adhesion to a polymeric matrix resin, and the material so treated.
2. The Prior Art
A composite consists of basically three major elements: the resin matrix, the reinforcement dispersed in the matrix, and the reinforcement-resin matrix interface. Synthetic fibers in staple or filamentary form, and fabrics produced therefrom, are known for polymer reinforcement. Typical of the fibrous reinforcements are glass, polyester, polyamide (nylon and aramid) and polyolefin fibers. Conventional matrix resins include thermoplastics, such as nylon and polyolefins, and thermosetting materials, such as epoxy and unsaturated polyester resins. Since the primary function of the fiber-matrix interface is to transmit stress from the matrix to the reinforcing fibers, the chemical and physical features of the interface are critical to the mechanical properties and end use performance of the composite. The compatibility between the reinforcing fiber and matrix is then a determining factor in the load sharing ability of the composite.
Polyolefin fibers are highly attractive reinforcing materials due to their low specific gravity, low raw material cost and good chemical resistance. It is known to prepare fibers on the basis of solutions of ultrahigh molecular weight polyolefins, particularly polyethylene, see U.S. Pat. No. 4,344,908, 4,411,854, 4,413,110, 4,422,993, 4,430,383, 4,436,689 and 4,551,296, all hereby incorporated by reference. However, polymeric matrix composites reinforced with these fibers suffer from poor fiber to resin adhesion.
U.S. Pat. No. 4,410,586, hereby incorporated by reference, discloses a method of producing a composite material in which a reinforcing polyolefin material is embedded in thermosetting or thermoplastic matrix material. The highly oriented polyolefin material, preferably a fiber or film having a draw ratio of at least 12:1, is treated prior to incorporation in the matrix by exposing its surface to a plasma discharge, preferably after etching with chromic acid. However, the degree of adhesion to resin for plasma treated fiber gradually reduces with time.
U.S. Pat. No. 2,999,056, hereby incorporated by reference, teaches the irradiation bonding of an unsaturated organic acid to a polymer substrate to enhance melt resistance. And ultraviolet irradiation has been shown, in batch studies, to induce graft copolymerization of monomers to polyethylene film.
The prior art fails, however, to teach the continuous, in line treatment of a polyolefin material to improve its compatibility with a resin which it reinforces, to thereby enhance the reinforced composite properties.